An accurate, quantitative method to measure the nerve fiber layer or retinal thickness is greatly needed as many eye diseases are directly related to changes in these thicknesses. Eye diseases can cause the retinal tissue and the nerve fiber tissue to thin. For example, diseases such as glaucoma can produce a loss of nerve fiber in the retina. In fact, a large portion of nerve fibers can be lost before any degradation is displayed in current test methods.
Besides nerve fiber or retinal tissue thinning, eye diseases can also cause a thickening of these tissues. For example, an increase in the nerve fiber layer can result from optic disc edema, of which papilledema is one of the most common forms. Also edematous diseases of the macular region can cause retinal thickness at and around the macula to thicken. In fact, one of the early warnings of visual impairment in diabetic retinopathy, is the apparent thickening of the retina. Thus, there is a need to measure accurately and quantitatively the retinal or nerve fiber layer thickness in order to diagnose and detect diseases early, to document these diseases' progress, to assess a disease's response to therapy and to aid in prescribing follow up treatment.
Prior attempts to measure the retinal and nerve fiber layer thickness have not resulted in any acceptable quantitative methods. Clinically, one obtains an impression of the retinal thickness by one of two methods, slit lamp biomicroscopy or stereophotography.
In the first method, a narrow beam of light is directed to the desired retinal location, and its intersection with the retina is viewed stereoscopically. The separation between the images from the surface of the retina and the pigment epithelium gives the clinician an estimate of the retinal thickness. This first method is subjective and not quantitative, depends on the angle between the viewer and the illumination, only provides a subjective indication of major changes in retinal thickness, and does not provide a permanent record that can be used for follow up treatment. The second method involves stereoscopic photography and stereo viewing under magnification. This second method also is not quantitative, is not sensitive, and is prone to the variability due to changes in magnification and the stereobase. An attempt at quantitation has been made using a stereoplotter to evaluate these stereophotographs, but this method requires very expensive instrumentation, unique operator skills, and is very time consuming.
An attempt to quantitatively measure the retinal thickness noninvasively has also been attempted using scanning ultrasonography. This method has not been clinically implemented yet, probably because it is limited by the fact that the location on the fundus from which the echoes are obtained cannot accurately be determined, and very small localized areas cannot be probed because the sonar beam's focal spot is greater than half a millimeter in diameter, and most importantly the measurement cannot be performed through the crystalline lens of the eye which scatters and absorbs the sonar wave.
At present, there are also no clinical methods available to measure the thickness of the nerve fiber layer. A qualitative method to evaluate the dropout or reduction of nerve fibers has been attempted. It is based on red free photography and the examination of the appearance of the nerve fiber layer surface. This method however, is subjective, not quantitative, and relatively insensitive to small changes of the nerve fiber layer.
A growing interest has been demonstrated in the measurement of the optic disc rim area portion of the retina where the nerve fiber layer comprises a significant amount of the retina layer. Such interest has been concentrated in using stereo photography and the digitization of images. However, this method may be inaccurate because it is based on an indirect evaluation of the nerve fiber layer, since the determination of the cup edge may depend (1) on the angle at which the nerve fibers bend as they come from the retinal area into the disc, and (2) on optical artifacts such as increased scattering and/or changes in color at the rim.
It is therefore the desire of this invention to noninvasively measure eye tissue components, such as the retinal and nerve fiber layer thickness by a method and apparatus that is quantitative, accurate, operator independent, not time consuming, reproducible, easily recordable, and inexpensive.